Dimethyl sulfoxide is widely used in industries as a solvent for polymerization of polymers and spinning polymer fibers. Additionally, recovery and reuse of dimethyl sulfoxide used once are widely performed in industry, and require steps of heating and distilling to purify.
It is, however, known that dimethyl sulfoxide is relatively thermally unstable and slightly decomposed when distilled under atmospheric pressure. In manufacturing or recovering dimethyl sulfoxide by distillation, contaminating a partially decomposed decomposition product in dimethyl sulfoxide reduces efficiency of dimethyl sulfoxide as a solvent. Thus, distillation of dimethyl sulfoxide is often performed under reduced pressure at 100° C. or less.
If dimethyl sulfoxide can be distilled, for example, at a high temperature of 110° C. or more, it is unnecessary to use high vacuum in distillation and, furthermore, no load is applied to a decompression device. Thus, facility for distillation can be simplified, which is industrially preferable.
There are known methods of adding a metal hydroxide such as sodium hydroxide or potassium hydroxide as an inhibitor for dimethyl sulfoxide decomposition (see JP-B-S43-3765, JP-B-S38-20721 and JP-A-2015-145359). The amount of addition of sodium hydroxide, potassium hydroxide or the like is limited to 0.003 to 0.5%, and adding in an amount of 1% or more promotes dimethyl sulfoxide decomposition.
In JP '765, the amount of a decomposition product quantified as formaldehyde after heating at 150° C. for 10 hours was 0.032% when no metal hydroxide was added, 0.054% when 1% of potassium hydroxide was added, and 0.052% when 1% of sodium hydroxide was added.
When purifying by distilling dimethyl sulfoxide, the purified dimethyl sulfoxide is distilled out of the system by the distillation. In addition, water contained in a dimethyl sulfoxide-containing liquid, a solvent having a lower boiling point than dimethyl sulfoxide, unreacted monomers in polymerization, and impurities such as a decomposition product of dimethyl sulfoxide are distilled out of the system by distillation. As a result, when a metal hydroxide such as sodium hydroxide or potassium hydroxide as an inhibitor for dimethyl sulfoxide decomposition is added, the metal hydroxide such as sodium hydroxide or potassium hydroxide remains at a bottom of a distillation column during purification as the distillation and purification of dimethyl sulfoxide proceed. This increases concentration of the metal hydroxide contained in a residual liquid after the distillation. In distilling dimethyl sulfoxide, even if a metal hydroxide is added at low concentration, dimethyl sulfoxide decomposition is rather promoted when dimethyl sulfoxide is distilled out and the concentration of the metal hydroxide reaches 1% or more. Thus, there has been a problem in that purity is reduced due to contaminating a decomposition product of dimethyl sulfoxide in distilled dimethyl sulfoxide.
There has been a desire for a dimethyl sulfoxide-purifying method that enables high purity dimethyl sulfoxide to be obtained safely, for example, at a high temperature of 110° C. or more and even when a decomposition inhibitor is highly concentrated in distilling and purifying dimethyl sulfoxide.
It could therefore be helpful to provide a method of purifying dimethyl sulfoxide to obtain high purity dimethyl sulfoxide.